Israeli PM said the strikes were a "precise counter-terror operation against an immediate threat".
Israel pounded Gaza with air strikes on Saturday and a Palestinian terrorist group retaliated with a barrage of rocket fire, in the territory's worst escalation of violence since a war last year.
Health authorities in the enclave controlled by Hamas said 10 people have been killed by Israel's bombardment, including a five-year-old girl, with 79 others injured. Israel's army estimated that its operation has killed 15 militants.
Israeli Prime Minister Yair Lapid said the Jewish state was forced to launch a "pre-emptive counter-terror operation against an immediate threat" posed by the Islamic Jihad group, following days of tensions along the Gaza border.
Israel and Islamic Jihad have both confirmed the killing of a top leader with the terrorist group, Taysir al-Jabari, in a Friday strike on a building in the west of Gaza city.
Islamic Jihad said the initial Israeli bombardment amounted to a "declaration of war", before it unleashed a flurry of more than 100 rockets towards Israel.
The rocket fire and Israeli strikes continued overnight, risking a repeat of an 11-day conflict in May 2021 that devastated Gaza and forced countless Israelis to rush to bomb shelters.
"Israel isn't interested in a wider conflict in Gaza, but will not shy away from one either," Lapid said in a nationally televised address.
Air raid sirens sounded overnight across several spots in southern and central Israel, but there were no immediate reports of casualties.
Officials in border areas urged people to stay close to shelters, which were also being opened in the commercial capital of Tel Aviv.
The army had initially confirmed at least 70 rocket launches from Gaza, saying 11 had fallen inside the strip, with dozens intercepted by the Iron Dome air defence system, and that others had landed in open areas.
Air raid warnings and Israeli strikes had at least temporarily abated by 4:30 am (0130 GMT).
Egypt, a historic broker between Israel and armed groups in Gaza, was seeking to mediate and may host an Islamic Jihad delegation later Saturday, Egyptian officials told AFP in Gaza.
Hamas has fought four wars with Israel since seizing control of Gaza in 2007, including the conflict last May. Islamic Jihad is a separate group, aligned with Hamas, but that also acts independently.
A flare up with Islamic Jihad came in 2019, following Israel's killing of Baha Abu al-Ata, Jabari's predecessor.
The sides exchanged fire for several days without Hamas joining the fray.
Hamas's moves amid the current violence could prove crucial in determining its severity, with the group facing pressure from some to maintain calm in order to improve economic conditions in the territory.
Gaza City resident Abdullah al-Arayshi said the situation was "very tense". "The country is ravaged. We've had enough of wars. Our generation has lost its future," he told AFP.
Hamas said Israel has "committed a new crime for which it must pay the price".
Flames poured out of a building in Gaza City following the first round of strikes, while wounded Palestinians were evacuated by medics.
Gaza's health ministry reported "a five-year-old girl, targeted by the Israeli occupation" was among nine people killed.
Five-year-old Alaa Kaddum had a pink bow in her hair and a wound on her forehead, as her body was carried by her father at her funeral.
Hundreds of mourners gathered in Gaza City for the funerals of Jabari and others killed in the air strikes.
Israeli military spokesman Richard Hecht said "we are assuming about 15 killed in action" in Gaza, referring to Palestinian combatants.
Israeli tanks were lined up along the border and the military said Thursday it was reinforcing its troops.
UN Middle East peace envoy Tor Wennesland said he was "deeply concerned", warning that the escalation was "very dangerous".
The strikes come four days after Israel closed its two border crossings with Gaza and restricted the movement of Israeli civilians living near the frontier, citing security concerns.
The measures followed the arrest in the occupied West Bank of two senior members of Islamic Jihad, including Bassem al-Saadi, who Israel accuses of orchestrating recent attacks.
Israel has been conducted a near relentless wave of raids inside the West Bank since mid-March in response to deadly attacks on Israeli citizens.
Islamic Jihad did not launch rockets after Saadi's arrest, but Israel has insisted through the week that the group was committed to seeking revenge.
Palestinians including patients and those with Israeli work permits have been prevented from leaving the Gaza Strip since Tuesday, while the goods crossing has also been shut.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)